Was a thread not long ago about it. But a bit like protecting your prize strawberry plants
Outwith a massive net or in this case cover, what ever method you use will only work for a short time and you've got to keep changing it up..
There is a way with thin fishing line and cds tied to them that works which they hate to land on, but like a shotgun, that very effective method is frowned on!
You could try a couple of kids windmills to create movement and a scarecrow.
The dummy owl or bird of prey is popular. but some say its got to be quite high above the birds to really work because instinct tells them that's where they get attacked from.
Recently on a YouTube video of Jim Davidsons boat he had a snake on his for keeping them off.
My favourite one is you can always just throw chips over the other boats well away from yours